Advanced Functions and Closures

This section explores some advanced features related to functions and closures, including function pointers and returning closures.

Function Pointers

We’ve talked about how to pass closures to functions; you can also pass regular functions to functions! This technique is useful when you want to pass a function you’ve already defined rather than defining a new closure. Functions coerce to the type fn (with a lowercase f), not to be confused with the Fn closure trait. The fn type is called a function pointer. Passing functions with function pointers will allow you to use functions as arguments to other functions.

The syntax for specifying that a parameter is a function pointer is similar to that of closures, as shown in Listing 19-27, where we’ve defined a function add_one that adds one to its parameter. The function do_twice takes two parameters: a function pointer to any function that takes an i32 parameter and returns an i32, and one i32 value. The do_twice function calls the function f twice, passing it the arg value, then adds the two function call results together. The main function calls do_twice with the arguments add_one and 5.

Filename: src/main.rs

fn add_one(x: i32) -> i32 {
    x + 1
}
fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
    f(arg) + f(arg)
}
fn main() {
    let answer = do_twice(add_one, 5);
    println!("The answer is: {answer}");
}

Listing 19-27: Using the fn type to accept a function pointer as an argument

This code prints The answer is: 12. We specify that the parameter f in do_twice is an fn that takes one parameter of type i32 and returns an i32. We can then call f in the body of do_twice. In main, we can pass the function name add_one as the first argument to do_twice.

Unlike closures, fn is a type rather than a trait, so we specify fn as the parameter type directly rather than declaring a generic type parameter with one of the Fn traits as a trait bound.

Function pointers implement all three of the closure traits (Fn, FnMut, and FnOnce), meaning you can always pass a function pointer as an argument for a function that expects a closure. It’s best to write functions using a generic type and one of the closure traits so your functions can accept either functions or closures.

That said, one example of where you would want to only accept fn and not closures is when interfacing with external code that doesn’t have closures: C functions can accept functions as arguments, but C doesn’t have closures.

As an example of where you could use either a closure defined inline or a named function, let’s look at a use of the map method provided by the Iterator trait in the standard library. To use the map function to turn a vector of numbers into a vector of strings, we could use a closure, like this:

fn main() {
    let list_of_numbers = vec![1, 2, 3];
    let list_of_strings: Vec<String> =
        list_of_numbers.iter().map(|i| i.to_string()).collect();
}

Or we could name a function as the argument to map instead of the closure, like this:

fn main() {
    let list_of_numbers = vec![1, 2, 3];
    let list_of_strings: Vec<String> =
        list_of_numbers.iter().map(ToString::to_string).collect();
}

Note that we must use the fully qualified syntax that we talked about earlier in the “Advanced Traits” section because there are multiple functions available named to_string. Here, we’re using the to_string function defined in the ToString trait, which the standard library has implemented for any type that implements Display.

Recall from the “Enum values” section of Chapter 6 that the name of each enum variant that we define also becomes an initializer function. We can use these initializer functions as function pointers that implement the closure traits, which means we can specify the initializer functions as arguments for methods that take closures, like so:

fn main() {
    enum Status {
        Value(u32),
        Stop,
    }
    let list_of_statuses: Vec<Status> = (0u32..20).map(Status::Value).collect();
}

Here we create Status::Value instances using each u32 value in the range that map is called on by using the initializer function of Status::Value. Some people prefer this style, and some people prefer to use closures. They compile to the same code, so use whichever style is clearer to you.

Returning Closures

Closures are represented by traits, which means you can’t return closures directly. In most cases where you might want to return a trait, you can instead use the concrete type that implements the trait as the return value of the function. However, you can’t do that with closures because they don’t have a concrete type that is returnable; you’re not allowed to use the function pointer fn as a return type, for example.

The following code tries to return a closure directly, but it won’t compile:

fn returns_closure() -> dyn Fn(i32) -> i32 {
    |x| x + 1
}

The compiler error is as follows:

$ cargo build
   Compiling functions-example v0.1.0 (file:///projects/functions-example)
error[E0746]: return type cannot have an unboxed trait object
 --> src/lib.rs:1:25
  |
1 | fn returns_closure() -> dyn Fn(i32) -> i32 {
  |                         ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
help: consider returning an `impl Trait` instead of a `dyn Trait`
  |
1 | fn returns_closure() -> impl Fn(i32) -> i32 {
  |                         ~~~~
help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
  |
1 ~ fn returns_closure() -> Box<dyn Fn(i32) -> i32> {
2 ~     Box::new(|x| x + 1)
  |
For more information about this error, try `rustc --explain E0746`.
error: could not compile `functions-example` (lib) due to 1 previous error

The error references the Sized trait again! Rust doesn’t know how much space it will need to store the closure. We saw a solution to this problem earlier. We can use a trait object:

fn returns_closure() -> Box<dyn Fn(i32) -> i32> {
    Box::new(|x| x + 1)
}

This code will compile just fine. For more about trait objects, refer to the section “Using Trait Objects That Allow for Values of Different Types” in Chapter 17.

Next, let’s look at macros!